Coating, material and corrosion

Corrosion is consumption of a material by chemical or electrochemical reactions with the environments to which the material is exposed. Generally, corrosion takes place when a metal is exposed in an environment of water/humidity and oxygen. Corrosion can effectively be prevented If one of the three parameters is removed. Securing dry environments underneath insulation, for instance, will decrease the risk of corrosion under insulation (CUI).

Coating a metal surface by means of a paint or another metal are common methods to prevent corrosion.

Cathodic protection may successfully be used for submerged surfaces. There are two common ways to obtain cathodic protection, either by the use of sacrificial anodes coupled to the metal to be protected, or by the use of impressed current between the metal and an auxiliary anode. In both cases the potential of the metal is forced towards a protection potential for the current metal. The method requires that the metal (cathode) and the anode are exposed in the same electrolyte.

Design optimisation and correct use of materials may further provide convenient solutions with regard to corrosion protection.

A corrosion cell consists of the following:

• Cathode (the most noble metal)
• Anode (the least noble metal)
• A metallic connection between the cathode and anode.
• An electrolyte (a liquid that conducts electricity )

The corrosion attack is affected by the following parameters:

• The environments (pollution, salts, acid/base)
• Temperature
• Metallic interconnection (risk of galvanic corrosion).

Seawater is more corrosive to steel than fresh water.

The galvanic series for seawater is a ranking of the metals and their ability to resist corrosion in seawater. In a coupling of two different metals, the metal with the most negative potential (less noble) will corrode while the metal with the most positive potential (most noble) will be protected. The greater potential distance between the two, the better protection.

As shown in the galvanic series below, graphite, platinum and titanium are most noble and hence most resistant to corrosion. The less resistant, as zinc, aluminium and magnesium are listed in the opposite end of the table.

Corrosion classes

ISO 12944-2 deals with the classification of the most important exposure environment for steel constructions.

• General/uniform corrosion
• Galvanic/ bimetallic corrosion
• Pitting
• Crevice corrosion
• Stress corrosion cracking
• Erosion corrosion
• Selective corrosion/leaching

General /uniform corrosion

Even corrosion attacks on the metal surface. Hence the corrosion rate can be estimated. May attack most metals. Examples are given in Fig. 1 and 2.

Galvanic corrosion

This is a type of corrosion that may occur by electrical contact between two dissimilar metals exposed in the same electrolyte. The most noble metal will act as a cathode and the less noble as an anode. The greater the potential difference between the two is, the higher the corrosion attack on the anode will be. A large cathode area and a small anode area will increase the corrosion rate. The risk of corrosion can be prevented by painting/ coating the noble part of the two. Examples are given in Fig. 3 and 4 below.

Pitting corrosion

Pitting corrosion occurs as small pits on the metal surface. In some cases the pits may perforate the metal thickness. The corrosion may attack most metals and especially some common stainless steels. AISI 316 stainless steel is susceptible to this type of corrosion when submerged in stagnant seawater. Examples are given in Fig. 5 and 6.

Crevice corrosion

Crevice corrosion often occurs in narrow crevices where liquid is trapped on a metal surface. As the oxygen content in the crevices decreases, corrosion attacks may be initiated. Crevice corrosion is an aggressive type of corrosion. Passive metals, such as stainless steels and aluminum are susceptible to crevice corrosion.

The corrosion may occur underneath gaskets, bolts/nuts, flanges, dirt, sand, loose paint and corrosion products. Examples age given in Fig. 7 and 8.

Stress corrosion cracking

Stress corrosion cracking (SCC) is the cracking induced from the combined influence of tensile stress and a corrosive environment. The required tensile stresses may be in the form of directly applied stresses or in the form of residual stresses. Cold deformation and forming, welding, heat treatment, machining and grinding can introduce residual stresses. An example is given in Fig. 9.

Erosion corrosion

Erosion corrosion is usually initiated by movements between a metal surface and a flowing corrosive medium. In severe cases solid components in a liquid may tear out particles from the metal, giving plastic deformation on the metal surface. This increase the activity of the metal. The result is a pattern with grooves or pits made by the flow direction and flowing conditions. Pipes, pumps, nozzles and valves are susceptible to this type of corrosion. An example is given in Fig. 10.

Selective corrosion / Leaching / Dealloying

In specific corrosive environments, some metal alloys can experience a type of corrosion where the least noble element in an alloy corrodes. This is known as selective corrosion, leaching or dealloying.

The result is a porous and mechanical weak material with low ductility ( the ability to be formed for instance by forging). A typical appearance is given in Fig. 11.

Pretreatment before painting

A thorough pretreatment is required to obtain a long life time for a construction. The first step is to grind sharp edges and welds and further remove contamination and irregularities introduced by mechanical rolling (as lamination) and welding (as weld fumes and spatter).

All contamination as dirt, oil, salts, dust and humidity must be removed. The best result is achieved by blast cleaning to Sa 2½ according to NS – EN ISO 8501 – 1, which gives the surface a favourable roughness and proper adhesion ability with regard to coating/painting.

Requirements for coatings / paints

• To be applied at certain climatic conditions
• Have to cure within a specified time.
• Shall meet the requirements of a specified colour. (for instance yellow escape routes)
• Shall meet the given corrosion protection requirements.
• Shall result in a cured coating /paint layer with the following characteristics:
  • Weather resistant
  • Proper hardness
  • Mechanical resistant
  • Chemical resistant
  • Diffusion tight (low permeability)
  • Expected colour and gloss

Corrosion protective paints are normally built up by three layers.

• Primer; secures a proper adhesion to the metal surface. In many cases corrosion preventive pigments are added.
• Medium layer; build up the coating thickness and gives a good barrier / corrosion protection..
• Top layer; gives the final colour and gloss, and often weather and chemical resistance.

Paints protect steel in three different ways

Barrier effect

• Gives a dense film on the steel surface
• Typical paints with this characteristic: alkyds, chlorinated rubbers, vinyls , epoxies , polyurethanes, polysiloxanes, polyesters, acrylics

Inhibiting (passivating) effect

• Reaction between the primer and the steel surface.
• Examples: zinc phosphate, (red lead paint)

Cathodic effect

• The pigments protect the steel by sacrificing themselves
• Examples: zinc ethyl silicate, zinc epoxy

Components in paint

• Binder - links the components of the paint. Provides characteristics like adhesion, weather resistance, chemical resistance and flexibility..
• Pigments - provides colour .
• Fillers - lingers the paints and give the required consistency.
• Solvents - dissolves the binder and gives the desired consistency before application.
• Additives - improve characteristics like sagging, skinning and foaming

The drying and curing mechanisms of paint

• Physically drying: Cures by evaporation of the solvents. (Acrylics, vinyls, chlorinated rubbers)
• Oxidative drying: Cures by reaction with oxygen. (Alkyds)
• Chemical drying : Cures by a chemical reaction (epoxies , polyurethanes, polyesters)

Coatings /paints usually applied on steel to protect against corrosion under insulation (CUI)

• Epoxies
• Epoxy phenolic
• Aluminum silicone
• Thermally sprayed coatings

Factors that may initiate corrosion under insulation (CUI)
Some types of insulation may lead to corrosion on steel when applied as first layer. Materials as AES fibers and mineral wool should not be applied as first layer unless the steel surface temperature is above 180°C. (At this temperature any water or humidity on the steel surface will evaporate), or if the insulation is separated from the surface by means of distance blocks and similar. (distance insulation).

The temperature is an important parameter in connection with CUI. A temperature range from 50 to 150°C increases the corrosion rate. An incorrect coupling of dissimilar metals, contamination on the steel surface and lack of maintenance also increases the risk of corrosion.

A closed cavity may be established underneath installed insulation. In lack of proper drainage, water and humidity may be trapped. Due to the concentration of salts, and the formation of aggressive chemical compounds, the corrosivity of the entrapped liquid will increase as function of time.

Carbon steels are generally susceptible to corrode in environments with a pH lower than 9. Titanium, which is a relatively noble metal is not negatively affected by neither acidic nor alkaline environments.

Situations contributing to CUI

• Lack of surface treatment
• Coating damage
• Worn coating
• Wrong type of coating
• Insulation with poor corrosion resistance installed directly on the steel surface.
• Humidity / water intrusion in the insulation.
• Insufficient drainage in the insulation system

The following circumstances may result in a leaky insulation systems, where humidity/water finds its way to the steel surface

• Sealing of the insulation system not according to the given requirements
• Lack of waterway
• Sealing cap/ jacketing joints and overlaps not according to the given requirements.
• Poor cap / jacketing workmanship around details
• Unfit cap / jacketing installed in weather exposed environments.
• The cap / jacketing is damaged
• Lack of drainage

Photos below illustrate leaky caps/jacketing

What types of corrosion may take place under insulation?

• General (uniform corrosion)
• Galvanic corrosion
• Pitting corrosion
• Crevice corrosion
• Stress corrosion cracking